The present invention relates to a magnetic reproduction system for performing magnetic reproduction utilizing changes in magnetic characteristics of a magnetic body which result from changes in a magnetic field formed by a magnetic recording medium.
A ring-type magnetic head is conventionally used for reproducing signals recorded on a magnetic recording medium. The magnetic head reproduces the electromotive force induced by a magnetic field formed by the magnetic recording medium. However, with this method, the reproduction output level and the S/N (signal-to-noise ratio) are largely dependent on the track width. Thus, this method is not suitable for high-density recording/reproduction. With a currently available ring-type magnetic head, the track width and the S/N are limited to 20.mu. and 43 dB, respectively.
In order to solve this problem, the present inventors have previously proposed (in Japanese Patent Disclosure (KOKAI) No. 57-36407) a magnetic reproduction system utilizing a new principle. This system incorporates a magnetic body which detects changes in a magnetic field formed by a magnetic recording medium as changes in the magnetic characteristics of a magnetic body, such as changes in magnetic permeability or high-frequency loss. According to this system, reproduction of signals is performed utilizing changes in the inductance of a coil wound around the magnetic body. The coil constitutes a tuned circuit together with a capacitance element. A high-frequency signal from a high-frequency oscillator is supplied to the tuned circuit. In this case, the magnetic characteristics change in accordance with a magnetic field formed by the magnetic recording medium. When the inductance of the coil wound around the magnetic body changes, the tuning frequency and the quality factor Q (resonance sharpness) of the tuned circuit change. Then, the high-frequency output signal from the tuned circuit changes. Such a change in the high-frequency output signal is detected by a detector, and a reproduction output corresponding to the signal recorded on the magnetic recording medium is obtained.
The above system can detect the slightest change in the magnetic field formed by the magnetic recording medium as a change in the characteristics of the magnetic body. Furthermore, since the reproduction output energy is supplied from a high-frequency oscillator, high-level reproduction may be performed with a high S/N. Satisfactory reproduction may be performed even if the track width is narrowed to 20.mu. or less.
Thus, the magnetic reproduction system described in Japanese Patent Disclosure (KOKAI) No. 57-36407 is suitable for high-density recording/reproduction, particularly as compared with other conventional systems. However, the recording density is still limited. In order to increase the recording density, the track width may be narrowed and the recording wavelength may be shortened. However, the width of the magnetic body must be decreased with such a decrease in the track width, and also the thickness of the magnetic body must be decreased with such a reduction in the recording wavelength. The thickness of the magnetic body must be decreased since the magnetic field formed by the magnetic recording medium strengthens toward the surface thereof and increasingly so with a decrease in the recording wavelength, that is, with an increase in the recording signal frequency. On the other hand, when the thickness of the magnetic body is increased, the magnetic field can only act on a smaller portion of the magnetic body, resulting in smaller changes in the magnetic characteristics of the magnetic body for the same degree of change in the magnetic field.
In this manner, the dimensions of the magnetic body must be reduced to the smallest possible in order to perform high-density recording/reproduction. Then, the dimensions of the coil to be wound around the magnetic body must also be reduced. When the dimensions of the coil are reduced, the quality factor Q is lowered due to an increase in the DC resistance. Since the Q of the tuned circuit is restricted by the Q of this coil, characteristic changes in the magnetic body due to the magnetic field formed by the magnetic recording medium may not be effectively converted into electrical signals. When the dimensions of the magnetic body are made very small, the ratio of the volume of the magnetic body to the effective volume of the coil, that is, the filling factor, is lowered, thereby degrading reproduction sensitivity. When the dimensions of the magnetic body are on the order of millimeters, the coil may be wound tightly around the magnetic body and the filling factor may be made approximately equal to 1. However, when the thickness of the magnetic body is significantly reduced to the order of microns, the filling factor in the direction of the thickness becomes 1/2 if the thickness of the coil wound on the magnetic body is also on the order of microns. When the thickness of the magnetic body is even further reduced to the order of submicrons, a decrease in the filling factor and reproduction sensitivity becomes even more pronounced.